Seismic exploration is one of the most powerful techniques for investigating the configuration of the rock strata beneath the earth's surface. The typical end product of a seismic survey is a map, termed a "seismic depth section", indicating the thickness and orientation of the various strata underlying that portion of the earth's surface from which the survey was conducted. By correlating the seismic depth section with other geologic information, such as data concerning surface outcroppings of various strata, wellbore cotings and logs, and previous seismic sections, surprisingly detailed information concerning the outermost several kilometers of the earth's crust can be developed. The predominant use of seismic exploration is in the search for subsurface structures favorable to the existence of oil and gas reservoirs.
Seismic reflection surveys, the most common type of seismic survey, are performed by initiating a shock wave at the earth's surface and monitoring at a plurality of surface locations the reflections of this disturbance from the underlying subterranean formations. These reflections occur from regions where there is a change in the acoustic impedance of the earth, generally the interface between adjacent strata. The devices used to monitor the reflections are termed geophones. The signal recorded by each geophone represents as a function of time the amplitude of the reflections detected by that geophone. To a good approximation, the reflections detected by each geophone occur from a point on each reflective surface located on a vertical line passing through the midpoint between the source and geophone. Thus, for every seismic disturbance ("shot"), each geophone records a signal ("trace") which represents features of the formations vertically beneath a known point on the surface of the earth.
In performing a seismic survey, a large number of geophones, usually between 48 and 1024, are positioned along the line of the survey. Accordingly, for each shot numerous traces are obtained. Each of the traces resulting from a single shot represents the reflections from the interfaces along a unique vertical line passing through the subterranean formations. At the time each trace is recorded, it is uniquely designated on the basis of source and detector position. In this manner, every trace is uniquely identified relative to all other traces. This information is later utilized in correcting and displaying the traces.
Following each shot the source is moved along the line of the survey and a second shot is made, yielding a new set of traces. Generally, the geophones are spaced equal distances apart and the movement of the source relative to the geophones is carefully established such that with the exception of one of the two end geophones, each source-detector centerpoint from the first shot corresponds to a source-detector centerpoint from the second shot. In the simplest type of seismic survey, this is accomplished by moving the source and detectors as a unit a distance equal to the geophone spacing following each shot. Continuing the survey in this manner, the numerous traces will each correspond to one of a smaller number of source-detector centerpoints. Thus, each centerpoint is represented by several traces. Traces having common centerpoints represent reflections occurring from the interfaces along a common line extending vertically downward from the centerpoint. Conducting the seismic survey such that each trace from a first shot corresponds to a trace from the next shot is termed common depth point surveying.
The traces obtained in performing the survey must be corrected prior to final display and analysis to compensate for various factors which impede direct comparison of the original traces. One of the most troublesome of these corrections involves compensating for the effects on the traces of an uppermost layer of the earth, typically 10-100 meters thick, termed the "low velocity layer" or "weathered layer." The velocity of seismic compressional waves (p-waves) through the low velocity layer is typically in the range of 500-1000 meters/second, while p-wave velocities in the strata below the low velocity layer are typically in excess of 1500 meters/second. Because the low velocity layer often differs greatly in thickness over relatively short horizontal distances, the transit time of a seismic wave through the low velocity layer can vary significantly over the line of a seismic survey. If not corrected for, this variation can significantly alter the observed configuration and depth of the underlying strata. For example, assuming horizontal bedding of the strata underlying the low velocity layer, a thin region in a low velocity layer of otherwise constant thickness can cause a portion of the horizontal strata to appear convex. Because even small variations in the calculated orientation of rock strata can have a major impact on decisions regarding the probability of oil and gas being found at a certain subterranean location, it is important that aberrations caused by the low velocity layer be determined with the greatest precision possible.
An early method of correcting for the effects of the low velocity layer is disclosed in U.S. Pat. No. 2,276,306, issued Mar. 17, 1942. In this technique, dynamite is used to initiate the seismic distrubance. Each dynamite charge is situated in the bottom of a hole drilled through the low velocity layer. A geophone situated near the mouth of the source hole records the vertical transit time through the low velocity layer at the same time a set of geophones spaced along the survey line records the reflections from the underlying strata. From the geophone located at the mouth of each source hole, the vertical transit time as a function of position along the line of the seismic survey is known. This permits the various traces recorded in the course of the survey to be corrected simply and accurately for the effects of the low velocity layer. In modern seismic exploration this method is rarely available since surface sources have largely replaced the use of subsurface sources due to the relatively great cost and environmental difficulties associated with the latter.
Another well known technique for establishing the effect of the low velocity layer on reflected seismic data is the intercept-time refraction method. In this method, for each shot the resulting traces are examined to determine the time required for the seismic wave to travel along a path from the source through the low velocity layer to the interface at the bottom of the low velocity layer along which it is refracted until received by the receivers. This refraction path is illustrated in FIG. 1. Because this is generally the fastest seismic path from source to receiver it is the first signal received by the receiver. For each source location, the refracted first arrivals are plotted for time as a function of source-receiver offset. A least squares fit is applied to the data to yield a straight line, the slope of which represents the refraction velocity V.sub.2, and having a specific intercept time, t.sub.r, corresponding to an offset distance of zero. By the application of Snell's law, it can be shown that the one-way vertical transit time through the weathered layer, t.sub.LVL, is given by the equation EQU t.sub.LVL =t.sub.r /2[1-(V.sub.1 /V.sub.2).sup.2 ].sup.1/2
where V.sub.1 =seismic P-wave velocity of low velocity layer
However, because the plot on which the intercept-time refraction method is based does not distinguish between the low velocity layer transit time at the source and the receiver, the vertical one-way transit time calculated from this method is an average of the two values. In modern seismic processing schemes the uncertainty introduced by this averaging in many instances represents a significant fraction of the total error in a stacked, corrected trace.
It would be desirable to provide a method of establishing static corrections for the low velocity layer in which the correction at each source location is independent of the low velocity layer at any receiver location and the correction at each receiver location is independent of the low velocity layer at any source location. It would be further desirable if this method of low velocity layer static correction did not require any special procedures or equipment in the field, depending only on that data normally acquired in the course of a common depth point p-wave seismic survey. It would be yet further desirable if this method yielded an absolute rather than relative correction for the low velocity layer.